It has long been realized that motionless mixers if made to work efficiently, provide certain economic advantages over dynamic mixers for, as the name implies, motionless mixers employ no moving parts. As such, motionless devices are generally less expensive to configure and certainly much less expensive to maintain while providing the user with an extended useful life for the mixer product in service.
Prior art approaches to motionless mixers have generally involved expensive machining, molding, casting or other fabrication of the component mixer elements coupled with some type of permanent attachment between elements and a conduit and/or between elements within a conduit. The resulting cost and difficulty of manufacture results in a relatively expensive end product. Moreover, many of the prior mixers provide less than complete mixing particularly with respect to material flowing along the walls of the conduit. This so-called "wall-smearing" is related to the parabolic velocity profile of a fluid having laminar flow in a pipe where the fluid velocity is small or zero along the wall surfaces.
Despite their limitations, static or motionless mixers are in common use in many industrial fields and are applied to both laminar and turbulent flow applications. A wide variety of mixing element designs is available from different manufacturers. Mixing elements are installed in a tube or pipe conduit in series and are fixed in position relative to the conduit wall. The cross-section is usually round but can be square or even rectangular. Materials introduced to the inlet or upstream side of the conduit on a continuous flow basis emerge mixed.
The number of mixing elements required to complete a given mixing task can range from two to twenty or more depending on the difficulty of the mixing application. In general, more mixing elements are required to solve laminar flow mixing problems than are needed in turbulent flow situations. One of the most difficult laminar flow mixing problems, for example, is to mix a small quantity of a low viscosity additive into a much higher viscosity main product flow. Mixing involves the application of the principals of distribution and dispersion. Referring to FIG. 1, it is seen that a small amount of an additive "A" is introduced on a continuous flow basis to a continuous main product flow "B". The two components then pass through a mixing system. The additive "A" is divided into many small components by the mixing system and the stream exits with the additive distributed across the cross-section of the main flow "B". The typical distance "S" between the concentration centers of the additive is small relative to the main flow diameter "D". Good distribution of additive "A" in stream "B" has been achieved.
The concept of dispersion is shown in FIGS. 2A and 2B. In FIG. 2A, the additive "A" is distributed in the main flow stream "B" material where molecular diffusion between "A" and "B" is virtually zero. The concentration values are either 0% or 100% or, in other words, the intensity "I" of "A" and "B" has a value of either 0% or 100%. In other words, zero dispersion has been achieved. However, in FIG. 2B some degree of molecular diffusion has occurred and the range of the intensity value found in the flow stream as measurements are taken across the conduit is now less than 0% to 100%.
It is obviously a goal in any mixing device to improve distribution and dispersion of component fluid streams. However, this is oftentimes difficult if this goal is attempted by simply adding more mixing elements. The addition of mixing elements often results in pressure drops across the mixing system while such systems tend to increase in length and cost to a point where such parameters prove prohibitive. Furthermore, small filament streams of component "A" can oftentimes tunnel through the mixing structure without further reduction in size.
Current motionless mixing designs use elements having a constant geometry in a given pipe size. In other words, the dimensions of scale of each mixing element relative to the pipe diameter does not change in proceeding through the motionless mixer in the direction of flow. As a result, increasing the number of mixing elements after a certain point has little effect on the mix quality of the output and, yet, as noted previously, additional mixing elements can create unwanted pressure drops across the system and resultant increased system costs. Even if the conduit cross-section were to be reduced in size in an attempt to improve mixing, substantial pressure drops can again result requiring additional pumping and resultant energy costs in carrying out the mixing process.
It is thus an object of the present invention to provide a motionless material mixing apparatus which improves upon the efficiency of mixing while not increasing the cost or pressure drop across the apparatus.